This invention relates to additive compositions, otherwise known as admixtures, for incorporation in hydraulic cement mixes, for example, hydraulic cement concretes, mortars, and grouts, neat cement mixes, nonplastic cement or concrete mixes, such as concrete block mixes, and dry mixes for making such concretes, mortars, and grouts.
Many admixtures employed to achieve improved compressive strength are known to act also as set retarders, as well as to reduce the quantity of water utilized in the hydraulic cement mix. In general, a reduction in the amount of water employed results in an increase in compressive strength. As retarders, such admixtures slow the chemical process of hydration so that the conrete remains plastic and workable for a longer time than concrete without such a retarder. Among the materials commonly used for improved compressive strength are the lignosulfonates, such as calcium lignosulfonate; salts of hydroxycarboxylic acids; sugars such as glucose (dextrose), maltose and the like; and highlypolymerized polysaccharides, such as dextrins.
While admixtures having set retarding and compressive strength improving properties are useful as set retarders per se, frequently there are instances where improved compressive strength is desired but any significant retarding of the rate of hardening of the cement or concrete mix would be undesirable. An example might be where concrete is to be placed during cold, especially freezing winter weather conditions. In such an instance, it is more desirable to have an accelerated setting, hardening, and strength development for the cement or concrete mix, as this will permit earlier removal of form work, speed up repair work, and reduce the time required for moist curing. To overcome the undesirable retarding effect, well known acceleration techniques that increase the rate of hydration for early strength development are frequently used to overcome the retarding tendency of the water reducer. Among the various techniques employed for this purpose are the increasing of the proportion of Portland cement in the mix; the use of the most rapid setting type of cement available; the heating of the water and other components of the concrete; and the use of chemical admixtures that act, catalytically or otherwise, to increase the rate at which the concrete hardens. Calcium chloride and alkanolamines, such as triethanolamine, are commonly employed as accelerating admixtures.
In those instances where the accelerator is added along with a strength increasing, but retarding admixture, the accelerator must be used in sufficient quantity that it neutralizes the retarding effect, or where needed, imparts an accelerating effect to the mix. This can lead to unpredictable results since the accelerator must be used in rather large quantities. Further, the use of chlorides can lead to a corrosion problem, which is an additional undesirable side effect, especially with prestressed concrete.
Other agents such as urea and calcium formate and the aforementioned alkanolamines, are not known to promote corrosion of metal, but have a less pronounced effect in accelerating the rate of hardening of concrete. Further, the use of low molecular weight aldehydes and polymers thereof, e.g., paraformaldehyde or Paraform, are known to strongly accelerate the rate of hardening of concrete containing Portland cement, but in aqueous solution such materials evolve esthetically objectionable and toxic fumes. As a result, the quantity of these materials that can be incorporated into concrete for set acceleration purposes without imparting such undesirable fumes to the concrete is very limited.
Thus a need exists for additive compositions, or admixtures, for incorporation in hydraulic cement mixes, which additives will provide improved compressive strength and/or accelerated rate of hardening and setting for the resulting cement products, while not causing adverse effects on the hydraulic mixes, such an unduly entraining air, or producing undesirable fumes or corrosive effects.